ISLAMABAD - Pakistan's top spy agency, Inter-Services Intelligence (ISI), has been accused of harassing witnesses appearing at the country's investigation into the United States military raid that last year killed al-Qaeda leader Osama bin Laden.

Witnesses at the Abbottabad Commission have come forward to say that the ISI would has been picking them up while traveling to and leaving the probe, often "briefing" them on what to say and grilling them afterwards on the proceedings.

Abbottabad Commission Head Justice Javed Iqbal has expressed anger at the alleged interference, describing it as meddling and establishing a committee of military officials to investigate the apparent harassment.

The matter first came to light when a witness from the neighborhood of Bin Laden's complex in Abbottabad was late for giving testimony. When the court demanded an explanation, he said that on his way to Islamabad, some ISI personnel stopped him and told him what to tell the commission.

Several other witnesses have now also alleged they were harassed by the ISI, both before and after recording their statements. The witnesses say while they wished to remain impartial in their statements, they also feared ISI reprisals.

Established on May 30, 2011 - just 28 days after the US raid - the Abbottabad Commission has quizzed hundreds of people on the events leading up to the operation, including civil and military officials. It is now compiling its final report

However, the leader of the opposition in the National Assembly, Chaudhry Nisar Ali Khan, had voiced serious misgivings about the commission's mandate, saying it was just formed to please the government and will produce no important findings.

In Pakistan's recent history, there have been numerous such commissions formed to investigate different incidents, but they rarely produce conclusive results.

A commission was formed to investigate the murder in March 2011 of Asia Times Online's Pakistan Bureau Chief Syed Saleem Shahzad, but it has so far produced no results, with no findings being made public.

A similar inquiry was formed in August 2011 to probe allegations that law enforcement authorities were involved in enforced disappearances in Balochistan. While the Supreme Court decreed in July that the disappearances should immediately stop and action taken against the culprits, there has been no evidence of this being implemented.

Critics say the Abbottabad Commission is equally unlikely to make major progress on the Bin Laden case.

A broader question raised by the ISI's alleged grilling of witnesses, is why the intelligence agency was seemingly so concerned with influencing its outcome.

There have been numerous reports since 9/11 claiming that the ISI helped hide Bin Laden, who was for over a decade the world's most wanted man. This was why the US special forces raid was launched without consulting Islamabad.

The complex where Bin Laden was found was located in close proximity to the Pakistan Military Academy, suggesting he was benefiting from close contacts with members of the army intelligence.

The ISI's international image has plummeted in recent months over the Bin Laden affair and its alleged links with the Afghan Taliban. By "briefing" witnesses in the Abbottabad Commission, the ISI likely sought to avoid the investigation implicating it in way - however an unlikely outcome that may be.

Malik Ayub Sumbal is an investigative journalist based in Islamabad, Pakistan. He has worked for more than nine years for a number of national and international newspapers, magazines, journals, wire services and television channels. He can be contacted at ayubsumbal@gmail.com